This invention relates to an imaging device, for example a large area image sensor formed using thin film technology on an insulating substrate or a display device such as a liquid crystal display (LCD) device, and in particular to such a device which incorporates protection against electrostatic discharge damage.
JP-A-2 61618 describes an imaging device comprising an array of imaging elements each having an associated switching element, data signal conductors connected to the imaging elements and switching signal conductors connected to the switching elements for enabling each imaging element to be accessed individually by selecting the associated switching and data signal conductors, reference conductor means and photosensitive protection means connected between the reference conductor means and the data signal and switching signal conductors for providing a low resistance path from the data and switching signal conductors to the reference conductor means when the photosensitive protection means are illuminated.
The device described in JP-A-2 61618 is an active matrix addressed LCD device in which the switching elements for controlling addressing of the imaging, in this case display, elements are in the form of thin film insulated gate field effect transistors (TFTs). The data and switching signal conductors are connected to the reference conductor means (which, as shown, consists of several reference conductors) by semiconductor films having a resistance which decreases when the films are illuminated. Thus, during manufacture of the device, the normally occurring illumination of the device will cause the semiconductor films to short the data and switching signal conductors to the reference conductors so inhibiting electrostatic damage which might otherwise occur during, for example, handling of the device. During electrical inspection of the device the semiconductor film is not illuminated, the data and switching signal conductors are thus not shorted and their electrical properties can therefore be tested. After the electrical inspection, the connection between the data and switching signal conductors and the reference conductors is removed by selectively etching the reference conductors.
The English language Abstracts for JP-A-62 198826 and JP-A-1 303416 similarly describe the use of photosensitive resistors made concurrently with the switching TFTs to connect the data and switching signal conductors to reference conductor means.
Although the use of such photosensitive resistors enables some electrostatic (ESD) protection, the resistance of the resistors may be too high to provide a satisfactory shorting of the data and switching signal conductors to the reference conductor means under illumination yet be sufficiently low when the resistors are not illuminated to make it necessary, as described in JP-A-2 61618, to selectively etch away parts of the reference conductors so as to avoid the possibility of undesired electrical paths in the completed and tested device. Also, because the photosensitive resistors will generally require a different composition of material from the imaging elements, extra masking and deposition steps will generally be needed to form the photosensitive resistors.